Polyethylene terephthalate is a thermoplastic polyester useful for various applications such as moldings, films, and fibers. While having a number of useful properties, the utility of this polyester for injection and extrusion molding applications would be enhanced if ways could be found to overcome certain deficiencies such as high deformation tendencies of molded parts when maintained under load at elevated temperatures. In its unreinforced state, polyethylene terephthalate has relatively low heat deflection temperatures (HDT) when measured under standard ASTM test conditions under constant load (usually at 264 or 66 psi).
U.S. Pat. No. 3,644,574 discloses blends of 1,4-butane diol polyesters with from about 1 to about 40 weight percent of a vinyl aromatic polymer (e.g., a styrenic polymer) which may be a copolymer, block polymer, graft polymer, or blend containing up to 50 weight percent of polymer units derived from one or more of the following: olefins containing up to 8 carbon atoms, vinyl naphthalene, divinylbenzene, alkyl esters of acrylic and methacrylic acids wherein the alkyl groups contain from 1 to 4 carbon atoms, acrylonitrile, methacrylonitrile, maleic anhydride, vinyl acetate, and other polymerizable compounds containing a ##STR1## and which give copolymers which are stable under the forming conditions. The blends referred to in the patent exhibit increased HDT as compared to the unblended polyester. The patent states that the improvements obtained with polybutylene terephthalate are not obtained when the polyester is polyethylene terephthalate.